The invention relates to a method for data transmission from a transmitting station--in particular a remote transmitting station--to a receiving station for processing the data, over several parallel lines, preferably optical fibers. The invention further relates to a device suitable therefor.
To be able to transmit several measured values or data words (bytes) within a certain transmission cycle, it is customary to assign to each measured value a transmission line of its own and to arrange the separate data bits of each byte in the form of a data message and to transmit them as a sequence of data pulses in such a way that a specific bit of the byte has assigned to it a specific data pulse position in the data message. To separate data messages successively transmitted on the same data line (which therefore extend in each instance over a number, corresponding to the number of bits), of data pulse positions, it has been customary until now to assign certain preset data pulse positions at the data telegram end and after a synchronization interval of a given minimum length, according to a recognition code.
The receiver samples the pulse positions at its data line terminal at a sampling rate corresponding to the pulse frequency and correlates the pulses picked up at these positions to the respective bits of the data word.
As a rule, the transmitter contains a high-frequency operating clock which controls the transmission clock for the pulse configuration of the data messages, while in the receiver an independent high-frequency operating clock controls the sampling of the data pulse positions. If there is no separate clock line for synchronization of the receiver sampling clock with the transmitter transmission clock, a synchronization interval is not only necessary for the separation of the data messages, but it is also utilized for compensating for phase shifts between the transmitter transmission clock and the sampling rate. Yet in such an asynchronous transmission method the number of usable data pulse positions within each data message is limited because of this asynchronism, and in addition a part of the available data pulse positions is occupied by the recognition code for message start and end.
According to U.S. Pat. No. 4,220,822, pulse trains of 64 clock pulses each are made available on a clock line for a time multiplexing method in an input/output device of a data processing system by a central pulse generator connected to the operating clock of the system. Data messages of 8 bits each are sensed successively, by eight transmitters connected to a common data line into eight receivers. To this end, in each transmitter the clock pulses are counted as a new pulse sequence begins. The transmitters with their associated receivers are successively activated only for the duration of a partial sequence of 8 clock pulses. Each partial sequence serves to synchronize the transmitter and receiver, while each whole pulse sequence controls the time-multiplex activation of the transmitter/receiver pairs. By appropriate programming, the length and the number of the data messages can be adapted to the practical case. But since the error rate increases with the length of the pulse sequences, such a time-multiplex method is trouble-prone, for example--at least at a higher repetition frequency of the clock pulses--for eight data telegrams of 15 bits each. Disturbances occur in particular during feeding in of bytes into a computer when these bytes are provided by a remote transmitting station over relatively long transmission lines at a higher transmission frequency, worth the receiving station arranged at the data inputs of the computer and containing a high-frequency clock pulse generator.